Deforestation: An anthropogenic cause of drought?

Oglesby et al. (2010), though admitting that natural drought was a known recurring feature of the Mayan area, highlighted the potential of anthropogenic deforestation in inducing warmer, drier drought-like conditions.

Using a climate model (MM5), various aspects of climate, including precipitation and temperature, were predicted from varying degrees of vegetation cover over the entire Mayan area. It was found that deforestation had two major effects (Figure 1). The first was that surface albedo increases, leading to a cooling and stabilisation of the atmosphere. The second was a large reduction in evapotranspiration from the surface, leading to warming and further stabilisation of the atmosphere. This is as a result of the energy once used for evapotranspiration being used to heat the surface, which then heats the air above it. As the atmosphere is stabilised, precipitation is reduced.   

Figure 1: Surface temperature differences (in ⁰C) for the MM5 simulation with all grassland minus all forested MM5 runs for (a) dry season and (b) wet season. Precipitation differences in (cm) for (c) dry season and (d) wet season (Source: Oglesby et al., 2010).

The biggest impacts were found to be in the wet season, where temperatures increased by 3⁰C-5⁰C and precipitation reduced by 15%-30%. Approximately 78% of the overall Maya region showed precipitation decrease, with a mean decrease of 17%. It is said that both the reduction in rainfall and increase in temperatures would have been detrimental to Mayan life. The reduction in rainfall means it would have been more difficult for the Maya to store enough water to survive the dry season, while the warmer conditions put more stress on evaporation, vegetation, livestock, and people.

The model’s results were validated and were found to be credible. However, it was also established that simulations of precipitation magnitude were less accurate, despite good predictions of spatial patterns.

Oglesby et al. (2010) hypothesised that the drought conditions that devastated the Maya resulted from a combination of natural variability and human activities. They state that neither the natural drought nor the human‐induced effects alone were sufficient to cause the collapse, but the combination created a situation the Maya could not recover from.

Oglesby et al. (2010) also point out that these results may have sobering implications for the present and future state of climate and water resources in Mesoamerica as ongoing massive deforestation is again occurring

This paper makes no headway in understanding what might have caused the Mayan collapse as no dated evidence assessed. However, it does highlight the potential of anthropogenic environmental degradation, and deforestation in particular, to strengthen natural drought.